I think of it like an N-body gravity simulation[1] with very very large N and no collision detection.

[1] http://justfound.co/gravity/

Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...

What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?

Actually, almost the only thing that we know about dark mater is that gravity affect it.

(Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)

Thanks!

If we can be wildly wrong about the number of red dwarfs couldn't we just as well be wrong about the number of brown dwarfs, rogue planets and other massive but dim/dark objects in the universe?

edit: Last year? More like 6 years ago. I feel old now. [0]https://www.cfa.harvard.edu/news/2010-26

But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?

Maybe think of it this way: expecting matter to be easily observable is an anthropocentric point of view, because human intuition defines matter as something that can be observed by the senses.

But the universe was not designed to be observed by us. There's no reason to expect that the majority of matter should be observable.

It turns out that in some ways we actually are precious snowflakes. What we are made off (baryonic matter) makes up less than 5% of the whole "stuff" of the Universe. A mere froth on the surface of existence.

And then some dark model builder says, "Guys, guys. I bet it's SU(3)xSU(2)xU(1) with 3 generations and the parameters tuned so that..."

How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.

Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint.

That's dark matter.

It's obviously bullshit, but it's the 'simplest' explanation.

"Imagine being told there's loads of bugs in your system and it's constantly breaking, and even though no-one can show you a single one of these bugs, tons of users are complaining."

That would fit the situation better: There's only one kind of indication that these things exist.

Edit: Do photons have gravity even though they are massless?

Edit2: Is that why gravity can bend photons?

GRT suddenly makes a bit more sense to me if the answer to these are 2x yes, so thank you!

Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of course, that is really the equivalence principle---the acceleration of all things under gravity's influence is the same. Whether or not light is a source of Newtonian gravity... I'm not sure. It's a tricky question because m=0. I want to say no because the "equal and opposite" forces should both be 0, even though one of them effects an acceleration on the other. I should emphasize that I'm not sure!

In Einsteinian gravity, the paths of photons (and indeed all things) are bent because spacetime itself is curved. Classical electromagnetic static fields and waves certainly have an energy density that can source gravity, and individual photons do too. But their energy is on the order of hbar. So you're talking a source of gravity like hbar/c^2. THIS IS REALLY TINY unless the photon's frequency is ENORMOUS.

Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hypernova directed at it - that we'll only know about when it hits us and wipes out our atmosphere. Well... light speed is far too slow for my taste ;).

Edit: It still doesn't answer my first question though: If you have something really energy dense like a neutron star - what fraction of gravity does energy make out then? 1E-3? 1E-10? 1/2? I'd find that interesting to know. According to wiki, neutron stars fall in temperature within years of creation from up to 1E12 K to 1E6 K. Six orders of magnitude. Depending on how much this decreases gravity I could imagine this effect alone influencing stellar orbits (I assume that a supernova would still allow other stars in a multi star system to continue existing). Has such a thing ever been measured?

Sorry.. there's just a whole can of worms opened about this in my head right now. Need to find an astro physicist to shake down :D.

    E = mc^2
    ∆m = ∆E/c^2

    ∆E = Q = mc∆T (c being specific heat)

    m = neutron star moles
    m = (mass of neutron star / mass of neutron) / Avogadro's #
    m = 2.9580163e33 mol

    c = 24 J / (mol * K)

    ∆T = 1e12 K - 1e6 K

    ∆E = 24 * 2.9580163e33 * (1e12 - 1e6) J = 7.0991681e45 J

    ∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg

But there are even more weird effects going on, due to the warping of gravity the mass of neutron stars can be up to 20% less than you'd expect based on its baryonic (neutron) constituents (questions 4 and 7):

https://www.astro.umd.edu/~miller/teaching/questions/neutron...

About your last equation, wouldn't J/m^2/s^2 come out as kg? 7.9E2 would be very low then, no?

I wonder how much energy is stored electromagnetically and through nuclear forces though. These things are supposed to have extremely strong EM fields and I imagine every piled up nucleus like a little atomic spring that has been depressed as much as possible. Wouldn't most of the stored energy be in there?

This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.

In that paradigm, considering a bulk of dark matter particles, the particles will be attracted to each other, and will fall towards the center of mass of the clump -- but there's nothing to stop them, and so they pass out to the other side of the cloud, where they decelerate. This puts a limit on how dense the cloud can become (I haven't studied the details of the mechanics here, but look into the Virial Theorem if you want the equations that describe these limits). In a normal cloud of gas in space, the particles would collide with something as they fall into the center of the cloud, which would convert their linear motion into random motion, and so they would essentially be trapped.

Note that while dark matter doesn't form dense objects, it does clump to some extent, and this is actually involved in galaxy formation [2]; based on initial small perturbations in the densities of matter before the inflationary period, the Cold Dark Matter forms clumps (halos) which act as the initial seeds of attraction for the baryonic matter (H/He) that formed the first galaxies.

[1]: http://www.ncbi.nlm.nih.gov/pubmed/10828999 [2]: https://en.wikipedia.org/wiki/Dark_matter_halo

So what do you mean by diffuse and could you point me to some source I can read up on this?

If it was concentrated in only specific spots it would cause different gravitational effects such as lenseing.

The lack of lenseing isn't the only issue it's also the general mass distribution across the galaxy for example the stars in the outer parts of the Milky Way move at nearly the same speed as the stars in the center. Since the center has much more mass the stars should move faster but they don't which means there is a lot of more mass that we do not see and that is distributed evenly across our own galaxy and not clustered in the center like the normal matter.

So to match the observation the dark matter has to be every where think of it like the air around you.

Now it doesn't have to be actual dark matter but it has to gravitationally affect the rest of the matter in the universe one of the version of string theory has dark matter as gravity leaking into our universe from higher dimensions or parallel universes the problem with that is that it still does not explain the diffusion unless the brain it's leaking from for some reason unlike our universe is diffused.

https://en.wikipedia.org/wiki/Gravitational_microlensing

You're right that all the dark matter in a galaxy would contribute to gravitational lensing around that galaxy, but that's a different signal; you'd be looking for the lensing at different scale, e.g. Einstein Rings around the edge of the galaxy, not inside it.

I meant 'diffuse' literally, in the physical sense, as in sparse, not dense -- the 'missing mass' that we're trying to explain is, under the dark matter hypothesis, spread out more thinly than if it was accounted for by dense dark objects like planets or stars.

Would the constructive interference between gravity waves be significant enough to account for 'dark matter'? And what would destructive interference look like (dark energy?)

Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8

But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?

https://en.wikipedia.org/wiki/On_shell_and_off_shell